STORMWATER, WASTEWATER, AND ENVIRONMENTAL PROTECTION 381To size the main and lateral sewers, use the Manning formula and the appro-
priateconveyance factorfrom Table 13.2.
When the conveyance factor Cfis used, the Manning formula becomes
(13.80)whereQflow rate through the pipe, ft^3 /s;Cfconveyance factor corre-
sponding to a specific nvalue listed in Table 13.3; Spipe slope or hydraulic
gradient, ft/ft.
Next, compute the flow rate for each lateral sewer discharging into the main
sewer using
LSfr(SSfr)/(percent lateral sewer flow, expressed as a whole number)
(13.81)In this formula, SSfris expressed in cfs, as computed earlier.
Compute the lateral sewer pipe diameter using the Manning formula, as given
above. Next, compute the value of for each lateral sewer pipe in the system.
Then take the sum of all lateral pipes, or
(13.82)
TheΣLSshould be less than d2.5for the main sewer. If theΣLSis greater than
the d2.5for the main sewer, the diameter of the main sewer should be increased
until its d2.5is greater thanΣLS.
Next, compute the sewer size with infiltration from the groundwater. Infil-
tration is the groundwater that enters a sewer. The quantity and rate of infiltra-
tion depend on the character of the soil in which the sewer is laid, the relative
position of the groundwater level and the sewer, the diameter and length of the
sewer, and the material and care with the sewer is constructed. With tile and
other joined sewers, infiltration depends largely on the type of joint used in the
pipes. In large concrete or brick sewers, the infiltration depends on the type of
waterproofing applied.
(^) LSd
2.5
LS1d
2.5
LS2....
d2.5LS
QCfS1/2
TABLE 13.3 Manning Formula Conveyance Factor
Pipe Pipe
diameter, cross-sectional
n
in (mm) area, sq ft (m^2 ) 0.011 0.013 0.015 0.017
6 (152) 0.196 (0.02) 6.62 5.60 4.85 4.28
8 (203) 0.349 (0.03) 14.32 12.12 10.50 9.27
10 (254) 0.545 (0.05) 25.80 21.83 18.92 16.70
12 (305) 0.785 (0.07) 42.15 35.66 30.91 27.27
15 (381) 1.227 (0.11) 76.46 64.70 56.07 49.48
18 (457) 1.767 (0.16) 124.2 105.1 91.04 80.33
21 (533) 2.405 (0.22) 187.1 158.3 137.2 121.1